SO2 removal is most often conducted by one of limestone slurry scrubbing, limestone addition to fluidized boilers, lime spray driers, caustic wet scrubbing or, more recently, by regenerable amine solution scrubbing. Each of these processes has properties, which tend to make them suitable for some particular applications but not others. Each specific situation is evaluated for the optimum choice of technology.
NOx (mainly nitric oxide NO, with low ppmv's of NO2) removal in many cases is done by Selective Catalytic Reduction (SCR) in which the NO is reduced to N2 by ammonia over a heterogeneous catalyst at elevated temperature. Deficiencies of this process are high cost, large equipment and the toxic and flammable nature of the ammonia reductant. Other means of NOx reduction which have been reported include processes which oxidize the NO to more water soluble NO2 or N2O5, which is then captured by an alkaline wet scrub. Oxidants such as chlorine dioxide, sodium hypochlorite, permanganate, hydrogen peroxide, ozone and corona discharge have been used. The cost of the reagents is generally prohibitive except for the conversion of small concentrations of NOx. The nitrates and nitrites, which are then captured in the subsequent wet scrubbing step present unacceptably high concentrations in the process effluent water in some cases.
U.S. Pat. No. 6,872,374 (Hakka et al) discloses a process that utilizes aqueous ferrous ethylenediamine tetraacetic acid chelates as agents for NO capture. The FeEDTA nitrosyl complex formed must be regenerated back to Fe2+ EDTA in order to reuse the complex reagent in absorbing further NO. The regeneration comprises a denitrosation step that occurs via a chemical reaction. Denitrosation can be accomplished by reacting the iron nitrosyl complex with sulfite and/or bisulfite, which is often present as a result of absorption of SO2 from the feed gas. This regeneration method produces so-called N,S products (e.g. iminodisulfonate and sulfamic acid salts). These must be removed from the solution to prevent accumulation.
Regeneration of an iron chelate absorbent by heating up the NO-containing solvent to drive off the NO has been reported (U.S. Pat. No. 4,158,044) but the pH of the solvent is between 2 and 3. However, it is known (J. Karhu, S. L. Alvarez Crespo, L. Harju and A. Ivaska, Preprints of the 1998 International Bleaching Conference, Helsinki, June 1-5, pp. 673-678) that the Fe2+ EDTA complex is unstable at these pH values, separating into insoluble EDTA in its acid form and iron ions, which bind only weakly to NO (U.S. Pat. No. 4,418,044). Other authors (R. J. Walker and H. W. Pennline, Paper No. 58d at the Symposium on Membrane Processes for Pollution Control, 1987 Annul Meeting of the AlChE, New York) report that they did not observe NOx in the off-gas from a thermal stripper.
A disadvantage of the use of iron chelates for NO capture is the high rate of oxidation of the ferrous chelate to the ferric form, which does not absorb NO. The Fe2+ ion oxidizes much faster when it is chelated to EDTA (K. D. Welch, T. Z. Davis and S. D. Aust; Archives of Biochemistry and Biophysics, 397. 2, pp. 360-369 (2002); D. C. Harris and P. Aisen, Biochimica et Biophysica Acta. 329 (1973) 156-158). The reduction of Fe3+ EDTA can be accomplished by the sulfite/bisulfite often present from the SO2 component of the feed gas, but this produces large quantities of dithionate salts, which must be removed from the solution to prevent accumulation. The use of other chemical reducing agents such as hydrazine (Archives of Environmental Protection, 24, 4, (1998) pp. 35-37; U.S. Pat. No. 5,200,160), ascorbic acid or dithionite (K. Smith, L. Benson, S. Tseng, M. Babu and P Bergman, Proceedings of the 1992 Clean Coal Conference) is also possible but the cost of the reagent makes the process uneconomical. Biological reduction can also be used (U.S. Pat. No. 5,891,408). Reduction by electrolysis has also been described (U.S. Pat. No. 5,320,816, U.S. Pat. No. 5,433,934 and U.S. Pat. No. 4,126,529), but the high duty required by the rapid oxidation of Fe2+ EDTA to Fe3+ EDTA again makes this alternative less desirable.
Flue gas produced by the firing of coal is a major source of mercury emissions into the atmosphere. Mercury is extremely toxic, affecting the nervous system. Since it tends to bioaccumulate into the food chain, even small concentrations can eventually cause health effects in humans and fauna. The concentration of mercury in flue gases is generally in the range of 10 micrograms per cubic meter, so effective capture can be difficult. Compounding this difficulty is the fact that the mercury is present both as particulates of ionic mercury(II) compounds and as a vapor of the elemental form. The ionic compounds can be captured by particulate collection devices such as baghouses, electrostatic precipitators (ESP's) or wet scrubbers but the vapor passes through freely. Processes for mercury vapor capture use one of two stratagems: either try to capture the mercury vapor (Hg°) as such by means of adsorbents such as activated carbon or by oxidizing the element to ionic mercury in the gas phase which can then be captured by means such as wet scrubbing. Suitable oxidants such as chlorine dioxide and ozone may be used. Deficiencies of the preceding mercury removal methods include high cost and insufficiently low removal efficiency.